This invention relates generally to an apparatus capable of continuous processing of articles, and more particularly it relates to a pre-heating chamber adapted for use in an apparatus for continuously processing a great number of optical elements such as lenses and mirrors at one time for successive application of a thin film coating on the opposite surfaces thereof which the apparatus is extremely reduced in dimensions by the provision of such pre-heating chamber.
One of the most common vacuum depositing techniques utilizes a bell jar of high vacuum in which a batch of substrates to be coated are simultaneously exposed to the vapor of material to be deposited as a thin film on the substrates. Since the bell jar must be opened after each depositing operation is completed in order to remove the processed substrates and replace them with a new batch of unprocessed substrates, the bell jar method is extremely time-consuming.
Continuous processing of substrates has been developed in the prior art to overcome the above-mentioned disadvantage of the bell jar method. For example, one method of continuous processing employs an entrance chamber, a pre-heating chamber, a depositing chamber, a cooling down chamber and an exit chamber connected in series by pass-ways having internal sealing means mounted therein through which substrates are advanced to coat the substrates with a layer of material. The substrates are mounted on individual work carriers, or vacuum deposition jig (hereinafter simply referred to as "jig") as illustrated in FIG. 1 wherein a number of optical elements 11 to be coated are mounted on a jig 10. After the jig is positioned in the entrance chamber, a vacuum source connected thereto is operated to evacuate the chamber. When the pressure in the chamber has reached a vacuum level of as high as 3 .times. 10.sup.-.sup.5 torrs at which the interiors of the other chambers, i.e., pre-heating chamber, depositing chamber and cooling down chamber also maintained, the jig is transferred from the entrance chamber to the pre-heating chamber in which the optical elements are heated by a radient energy heater to a predetermined temperature, e.g., approximately 200.degree.-300.degree.C for a period of time, e.g., approximately 20-40 minutes, these conditions being dependent upon the material of optical elements to be coated. The pre-heating of the substrates drives off adsorbing gases and volatile surface contaminates to clean the substrate surfaces and render them suitable for receiving the deposition coating strongly adherent thereto, thus being essential in the vacuum deposition coating processes. From the pre-heating chamber, the jig is transferred into the depositing chamber to apply a coating of a predetermined thickness on either of the surfaces of each of the elements while rotating the jig usually in a horizontal plane to effect uniformity of thin film characteristics. Upon completion of vacuum depositing operation, the jig is advanced past the cooling down chamber to the exit chamber from which it is removed to outside.
In such a process, the depositing operation requires about 1-2 minutes for application of a monolayer coating and about 4-5 minutes per layer for application of a multilayer coating, while the pre-heating treatment requires 20-40 minutes, occupying a large proportion of the period of time necessary for one cycle of processing operation, or the time interval between the feeding of a jig and the removing of the processed jig, thereby the efficiency of the apparatus is extremely reduced. To effect an increase in the efficiency, the depositing chamber may be provided with a necessary number of additional pre-heating and cooling chambers connected in parallel thereto. But this complexity makes the resulting apparatus quite bulky.
For successive applications of a coating on the opposite surfaces of a substrate, after the substrate is processed in a processing chamber to apply a coating on either of the surfaces thereof, the once processed substrate is removed from the processing chamber into the ambient atmosphere where the substrate is inverted, and then the inverted substrate is inserted in the same processing chamber in which an identical processing operation is repeated to apply a coating on the opposite surface thereof. Such a double processing procedure also requires a considerably long processing time. In addition, a drawback encountered by the necessity of inverting the once processed substrate in the ambient atmosphere is such that since the substrate as well as the processing chamber are susceptible to contamination by the surrounding atmosphere and from the fragments of coating material adhering to the jig framework, impurities may accumulate on the substrate surface to be coated subsequently. Therefore it is necessary to incorporate into an operating cycle for the double processing an additional pre-cleaning treatment prior to the second depositing treatment, or otherwise it is difficult to control the properties of the deposited film produced by different depositing operations in the same apparatus.
One conventional double processing method utilizes a bell jar in which a jig is placed in a vertically disposed position between two boards containing a material to be deposited, from which the vapor of the material is allowed to deposit on the opposite surfaces of the substrate simultaneously. Since application of a coating is made on the opposite surfaces of the substrate in one time, the processing time is comparatively short. However, in order to make the thickness of the coating deposited on the substrate uniform, it is more desirable to set the jig in the horizontally disposed position during the depositing operation.